I found a day planner in a clearance bin for 1$ and have been keeping a log of each tank in it. I keep logs of water tests, new additions, deaths, splittings, spreadings. This way I know how old each inhabitant is and how many times the corals have spread and multiplied. I track my fragging ventures. It's just a notebook and I plan on putting it in an excel file sooner or later but it helps me keep track and notice when things are getting off kilter.